Dispersion-compensated optical wavelength router and cascaded architectures

ABSTRACT

An optical wavelength router includes a beamsplitter, a first resonator, and a second resonator. The beamsplitter separates an input signal into a first beam and a second beam. The first resonator reflects the first beam and has a partially reflective front surface and a highly reflective back surface spaced a first optical thickness from the front surface. The second resonator reflects the second beam and has a partially reflective front surface and a highly reflective back surface spaced a second optical thickness from the front surface. The difference between the optical thicknesses of the first and second resonators is approximately equal to one-eighth wavelength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.09/798,659, filed Mar. 1, 2001, by Gan Zhou and Kuang-Yi Wu.

FIELD OF THE INVENTION

The present invention relates generally to the field of opticalcommunications systems. More specifically, the present invention relatesto a dispersion compensated optical wavelength router and cascadedarchitectures.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing is a commonly used technique thatallows the transport of multiple optical signals, each at a slightlydifferent wavelength, over an optical fiber. The ability to carrymultiple signals on a single fiber allows that fiber to carry atremendous amount of traffic, including data, voice, and digital videosignals. For example, the International Telecommunications Union (ITU)Draft Recommendation G.mcs proposes a frequency grid which specifiesvarious channel spacings including 100 GHz and 200 GHz. It would beadvantageous to obtain smaller channel spacings. As transmission systemsevolve to longer distances, smaller channel spacings, and higher bitrates, however, the phenomenon of dispersion becomes a limiting factor.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an optical wavelength routerthat includes a beamsplitter, a first resonator, and a second resonator.The beamsplitter separates an input signal into a first beam and asecond beam. The first resonator reflects the first beam and has apartially reflective front surface and a highly reflective back surfacespaced a first optical thickness from the front surface. The secondresonator reflects the second beam and has a partially reflective frontsurface and a highly reflective back surface spaced a second opticalthickness from the front surface. The difference between the firstoptical thickness and the second optical thickness is approximatelyequal to one-eighth wavelength.

In another embodiment of the present invention, an optical devicecomprises a first stage optical wavelength router and a second stageoptical wavelength router. The first stage optical wavelength routerreceives an input wavelength division multiplexed signal and generates afirst output signal comprising a first subset of wavelength channelsfrom the input signal and a second output signal comprising a secondsubset of wavelength channels from the input signal. The first stageoptical wavelength router is characterized by a chromatic dispersionprofile having a first frequency offset. The second stage opticalwavelength router receives the first output signal and generates a thirdoutput signal and a fourth output signal. The second stage opticalwavelength router is characterized by the chromatic dispersion profilehaving a second frequency offset such that the difference between thefirst frequency offset and the second frequency offset comprisesone-half of the period of the chromatic dispersion profile.

The following technical advantages may be achieved by some, none, or allof the embodiments of the present invention. The optical wavelengthrouter performs a multiplexing and/or a demultiplexing function togenerate output waveforms that have a flat-top passband, good isolation,and very low chromatic dispersion. These and other advantages, features,and objects of the present invention will be more readily understood inview of the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction withthe accompanying drawings, in which:

FIGS. 1A and 1B illustrate one embodiment of an optical wavelengthrouter according to the present invention;

FIG. 2 illustrates the optical wavelength router arranged in a tiltedconfiguration;

FIG. 3 illustrates an example of the spectral response of the opticalwavelength router;

FIG. 4A illustrates one embodiment of a resonator that may be used inthe optical wavelength router;

FIG. 4B illustrates another embodiment of a resonator using an air-gapstructure;

FIG. 5 illustrates the phase functions for the two arms of the opticalwavelength router;

FIG. 6 illustrates the waveform, group delay, and dispersion of theoptical wavelength router;

FIG. 7 illustrates a conceptual diagram of an interferometer;

FIG. 8 illustrates a block diagram of a Michelson interferometer with aresonator;

FIG. 9 illustrates the phase functions for the two arms of theinterferometer illustrated in FIG. 8;

FIG. 10 illustrates the waveform, group delay, and dispersion of theinterferometer illustrated in FIG. 8;

FIG. 11 illustrates another embodiment of the optical wavelength routeraccording to the present invention;

FIGS. 12A and 12B illustrate one embodiment of a Faraday rotator;

FIG. 13 illustrates yet another embodiment of the optical wavelengthrouter according to the present invention;

FIG. 14 illustrates a cascaded architecture of optical wavelengthrouters;

FIG. 15 illustrates an optical networking architecture using the opticalwavelength routers;

FIGS. 16A and 16B illustrate dispersion profiles for alternativeembodiments of an optical wavelength router according to the presentinvention;

FIG. 17 illustrates a dispersion profile for various stages of oneembodiment of a cascaded architecture;

FIG. 18 illustrates a dispersion profile for various stages of anotherembodiment of a cascaded architecture; and

FIG. 19 illustrates a dispersion profile for various stages of a furtherembodiment of a cascaded architecture.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates one embodiment of an optical wavelength router 10that includes a beamsplitter 20 and resonators 30 a and 30 b. Resonators30 a and 30 b are collectively referred to as resonators 30. In general,router 10 performs a multiplexing function and/or a demultiplexingfunction and reduces the dispersion generally associated with performingthese functions. When performing the multiplexing function, router 10combines two streams of optical signals into a single, more denselyspaced signal stream. The multiplexing function of router 10 isdescribed in greater detail below. When performing the demultiplexingfunction, router 10 separates a dense signal stream into two, widerspaced streams. For example, beamsplitter 20 of router 10 receives aninput signal 12 at an input port and splits signal 12 into a first beam14 propagating along a first optical path and a second beam 16propagating along a second optical path. The beams 14 and 16 propagatingalong each path are reflected back by the appropriate resonators 30 aand 30 b. The two reflected beams combine and interfere at thebeamsplitter 20 to form a first output signal 22 and a second outputsignal 24. Output signal 22 back-propagates toward the input and exitsat an output port A. Output signal 24 emerges from an output port B.

Input signal 12 comprises a WDM signal containing multiple opticalchannels to define an input spectral band. The outputs signals 22 and 24emerging at output ports A and B contain two complementary subsets ofthe input spectral band such that, for example, output signal 22comprises a WDM signal containing the even channels of the inputspectral band and output signal 24 comprises a WDM signal containing theodd channels of the input spectral band. Therefore, alternating opticalchannels in the input spectral band are routed to each output port(e.g., even channels are routed to output port A, and odd channels arerouted to output port B), as shown in the graph provided in FIG. 3.Router 10 therefore performs a demultiplexing function. If desired, thisrouter 10 can be extended in a cascaded architecture with multiplestages of optical routers 10 to progressively separate individualchannels or groups of channels. A description of a cascaded architectureis detailed with respect to FIG. 14.

For simplicity of discussion, beamsplitter 20 is illustrated in FIG. 1Aas a non-polarizing beamsplitter cube. The beam splitting takes place ata surface 40, and the four outer surfaces of the beamsplitter 20 arecoated with anti-reflection film. It should be noted that generally thebeamsplitter 20 shown in FIG. 1A can either be polarization-based ornon-polarizing. For example, the beamsplitter 20 can be a non-polarizingor polarizing thin film beamsplitter, a birefringent beam displacer, adiffractive optical element, or an optical coupler.

Resonator 30 comprises a cavity with a partially reflective frontsurface and a totally reflective back surface, such as, for example, anetalon. FIGS. 4A and 4B illustrate examples of resonators 30 that may beused in router 10. It should be understood, however, that other types ofresonators 30 may be used to achieve the unique features and functionsof the present invention.

FIG. 4A illustrates one embodiment of a single cavity Gires-Tournoisresonator having two mirror surfaces 34 and 35 separated by intermediatematerial 32. The surfaces 34 and 35 are parallel to each other. Thefront mirror 34 is partially reflective, while the back mirror 35 ishighly reflective. Consistent with standard terminology in the art, theoptical thickness, d, of a resonator 30 is defined as the physicalthickness of the gap 32 multiplied by the refractive index of theintermediate material 32.

FIG. 4B illustrates another embodiment of a single cavity Gires-Tournoisresonator having two mirror surfaces 34 and 35 parallel to each otherand separated by an air gap 32. The layers 31 and 33 are transparent.The front surface 36 of the first layer 31 can be coated withanti-reflection film. Typically, the surface 36 also has a wedge anglerelative to the mirror surfaces 34 and 35 to further reduce the effectof residual reflections from the surface 36. The optical thickness ofthe resonator here is the physical thickness of gap 32 multiplied by therefractive index of air. In general, the optical thickness of an objectis equal to its physical thickness multiplied by the refractive index ofthe material forming the distance.

Returning to FIG. 1A, the optical thicknesses of the resonators 30 a and30 b are referred to as d1 and d2, respectively. The amplitudereflectivities of the front mirrors of the resonators 30 a and 30 b arereferred to as r1 and r2, respectively. The wavelength router 10illustrated in FIG. 1A has two arms. The first arm traces the beam 14propagation path toward resonator 30 a and the second arm traces thebeam 16 propagation path toward resonator 30 b. For example, the firstarm starts at the point of interception between the input beam 12 andsurface 40 of beamsplitter 20. It includes the upper-left half of thebeamsplitter 20, followed by the gap between the beamsplitter 20 andfirst resonator 30 a, then the resonator 30 a. The optical path lengthof the first arm is referred to as L1, and it is defined as thesummation of the optical thicknesses of all the parts in this armincluding the first resonator 30 a. The optical path length of thesecond arm, L2, is defined similarly. The interferometer path lengthdifference ΔL is defined as (L2−L1).

In operation of router 10 performing a demultiplexing function,beamsplitter 20 splits input signal 12 into beams 14 and 16. If thebeamsplitter 20 is a polarization beamsplitter, beams 14 and 16 willhave orthogonal polarizations. Beams 14 and 16 are directed ontoresonators 30 a and 30 b, respectively. Each beam 14 and 16 striking thepartially-reflective layer 34 of a resonator 30 is partially transmittedthrough the partially-reflective layer 34 into the resonator cavity 32,and is then reflected by the reflective layer 35 through thepartially-reflective layer 34 toward the beamsplitter 20. A portion ofthe each beam 14 and 16 is also reflected back by thepartially-reflective layer 34 along its optical path toward thebeamsplitter 20 without propagating through a resonator 30. Eachresonator 30 reflects substantially all of the incident optical powerback regardless of wavelength, but the group delay of the reflectedbeams is strongly dependent on wavelength.

Both of the reflected beams from the resonators 30 a and 30 bback-propagate along their respective optical paths toward thebeamsplitter 20, where they are combined and interfere to produce outputsignals 22 and 24 containing complementary subsets of the input spectralband. For example, output signal 22 comprises a first subset of theinput spectral band, such as the even channels of input signal 12. Inthis example, output signal 24 comprises a second subset of the inputspectral band complementary to the first subset, such as the oddchannels of input signal 12. Output signal 22 emerges from router 10 atoutput port A while output signal 24 emerges from router 10 at outputport B. Therefore, when performing the demultiplexing function, router10 separates a dense signal 12 into two, wider spaced signals 22 and 24.Router 10 achieves low dispersion in this endeavor using resonators 30 aand 30 b.

FIG. 1B illustrates the operation of router 10 performing a multiplexingfunction. Input signals 50 and 52 contain complementary subsets of anoutput spectral band. Beamsplitter 20 splits each input signal 50 and 52into beams 54 and 56 which are directed onto resonators 30 a and 30 b,respectively. Beam 54 contains components of both signal 50 and signal52. Similarly, beam 56 contains components of both signal 50 and signal52. Each beam 54 and 56 striking the partially-reflective layer 34 of aresonator 30 is partially transmitted through the partially-reflectivelayer 34 into the resonator cavity 32, and is then reflected by thereflective layer 35 through the partially-reflective layer 34 toward thebeamsplitter 20. A portion of each beam 54 and 56 is also reflected backby the partially-reflective layer 34 along its optical path toward thebeamsplitter 20 without propagating through a resonator 30. Eachresonator 30 reflects substantially all of the incident optical powerback regardless of wavelength, but the group delay of the reflectedbeams is strongly dependent on wavelength.

Both of the reflected beams 54 and 56 from the resonators 30 a and 30 bback-propagate along their respective optical paths toward thebeamsplitter 20, where they are combined and interfere to produce outputsignal 58. Output signal 58 generally defines an output spectral bandcomprising each of the complementary subsets of channels in inputsignals 50 and 52. Therefore, when performing the multiplexing function,router 10 combines two streams of optical signals 50 and 52 into asingle, more densely spaced signal stream 58. Router 10 achieves lowdispersion in this endeavor using resonators 30 a and 30 b.

FIG. 2 illustrates the optical wavelength router 10 of FIG. 1A in atilted configuration. In particular, resonator 30 a is arranged at abias angle Θ_(a) with respect to the normal of the optical path of beam14. Resonator 30 b is arranged at a bias angle Θ_(b) with respect to thenormal of the optical path of beam 16. In general, Θ_(a) and Θ_(b) areeach set at an angle from 0.5 to 10 degrees to achieve an appropriatetilt configuration of resonators 30. In a particular embodiment, Θ_(a)and Θ_(b) are each set at approximately the same angle. In operation,each of beams 14 and 16 reflected by resonators 30 a and 30 b,respectively, propagates toward beamsplitter 20 along an optical paththat is offset from its original optical path toward resonators 30 a and30 b. As a result, optical signals 22 and 24 emitted by router 10 areisolated from input signal 12. It should be understood that the tiltconfiguration of resonators 30 a and 30 b is not limited to thatillustrated in FIG. 2. Rather, any tilt configuration of resonators 30 aand 30 b suitable to isolate output signals 22 and 24 from input signal12 is contemplated. Moreover, router 10 illustrated in FIG. 2 may alsobe operated in a multiplexing function, as described above with regardto FIG. 1B, while still achieving isolation of input and output signalsand low dispersion.

FIGS. 5 and 6 illustrate the performance characteristics of wavelengthrouter 10. This example demonstrates the construction of alow-dispersion, 50 GHz optical demultiplexer (i.e., the input channelsare spaced 50 GHz apart, and the output channels are 100 GHz apart).First, the optical thickness d2 of resonator 30 b is selected so thatresonator 30 b has a free spectral range (FSR) of approximately 50 GHzand the resonance frequencies are at f_(c)+/−25 GHz. Here f_(c) denotesthe center frequencies of the WDM channels of input signal 12 that arespaced, for example, 50 GHz apart. The FSR of resonator 30 b here isdefined as the period of the resonator's complex reflectivity.

The above conditions are achieved by following the equation:d2=(m/2)*λ_(c)+(¼)_(.)*λ_(c)and picking the integer m such that the equation:d2=c/(2*FSR)is satisfied to best approximation. Here λ_(.c) is the center wavelengthof any one of the input channels within the FSR of the particularresonator 30; and c is the speed of light in a vacuum. In a particularembodiment, λ_(.c) is the center wavelength of the center input channelswithin the FSR of the particular resonator 30. In an example for ac-band 50 GHz router 10, we can use λ_(.c)=1545.32 nm, c=2.99792458*10⁸M/sec, and therefore d2=2.998307 mm. The optical thickness d1 ofresonator 30 a is set such that d1=d2+/−(¼)_(.)*λ_(c). In the examplewhere d1=d2−(¼)_(.)*λ_(c), d1=2.997921 mm.

By following the procedure above, the center wavelength of the resonator30 a is offset relative to the center wavelength of the resonator 30 bby approximately one half of the free spectral range of both theresonators. For example, if the free spectral range of both resonatorsis approximately 50 GHz, then the center wavelength of resonator 30 a isoffset by approximately 25 GHz relative to the center wavelength ofresonator 30 b. This causes the resonance frequencies of the resonator30 a to match that of the anti-resonance frequencies of the resonator 30b. As will be elaborated further below, this arrangement of theresonators' center wavelengths can significantly reduce chromaticdispersion in the device, while keeping a flat-top passband and goodisolation.

The back mirror reflectivities of the two resonators 30 are both set tobe 100%. The front mirror reflectivities of the resonators 30 can bevaried to adjust the passband, isolation, and dispersion of theinterleaver waveform. In the example of FIG. 5 and FIG. 6 the frontmirror reflectivities are set at r2=0.2 (i.e., 4% reflectivity) andr1=0.12 (i.e., 1.44% reflectivity). Finally, the interferometer pathlength difference, ΔL, is set to be approximately (½)*d2, which comesout to ΔL≈1.499 mm.

A technical advantage of wavelength router 10 is its low dispersion.This can be most readily understood by comparison to a conventionalMichelson interferometer in which an incoming optical beam is split50/50 between two optical paths (e.g., by a beamsplitter), as shown inFIG. 7. The beam propagating along the first path experiences a phaseshift, Φ1(f). Similarly, the second beam experiences a phase shift,Φ2(f). Note that f denotes the optical frequency, and that both thephase shift functions are frequency (or wavelength) dependent. The twooutput optical fields of the interferometer can be written as:Ea=exp(−iΦ1)+exp(−iΦ2)andEb=exp(−iΦ1)−exp(−iΦ2)After some algebra, the two fields can be rewritten as:Ea=2 cos [−(Φ1−Φ2)/2] exp [−i(Φ1+Φ2)/2]Eb=−2 sin [−(Φ1−Φ2)/2] exp [−i(Φ1+Φ2)/2]The key result from the above analysis is that the output waveform fromthe interferometer depends on the phase difference between the two arms.In contrast, the overall phase shift, and therefore the dispersionproperty, depends on the sum of the two phase functions. In mathematicalterms:Waveform ∝ cos [−(Φ1−Φ2)/2]² or sin [−(Φ1−Φ2)/2]²Group Delay ∝d(Φ1+Φ2)/dfDispersion ∝d ²(Φ1+Φ2)/df ²

FIG. 8 shows an interferometer 100 in which an input signal 102 is splitinto two beams by a beamsplitter 110. One beam propagates toward amirror 120 and is reflected back by this mirror 120 toward thebeamsplitter 110. The other beam propagates toward a resonator 130 andis also reflected back toward the beamsplitter 110. The resonator 130 isa cavity with a partially-reflective front mirror and atotally-reflective back mirror, as shown for example in FIGS. 3 and 4.The resonator 130 reflects back substantially all of the incidentoptical power regardless of wavelength, but the group delay of thereflected light is strongly dependent on wavelength. The two reflectedbeams from the mirror 120 and from the resonator 130 interfere at thebeamsplitter 110 and the resulting output is split into two outputsignals, one at output Ea, and the other in a different direction atoutput Eb. The two output signals contain complementary subsets of theinput spectral band. The two output ports Ea and Eb divide the spectralspace evenly with alternating optical channels being directed to eachoutput port (i.e., odd optical channels 1, 3, 5, 7, etc. are directed tooutput port Ea, while even channels 2, 4, 6, etc. are directed to outputport Eb). Such a device concept has been proposed by B. B. Dingle and M.Izutsu, “Multifunction Optical Filter With A Michelson-Gires-TournoisInterferometer For Wavelength-Division-Multiplexed Network SystemApplications,” Optics Letters, vol. 23, p. 1099 (1998) and thereferences therein.

FIG. 9 shows the corresponding phase functions of the two arms of theinterferometer 100. Φ1 is the phase function of the resonator arm and Φ2is the phase function (i.e., a straight line) of the mirror arm. Thephase difference shows a step-like behavior with a distance of π betweensuccessive flat regions. This explains why the waveform is theflat-topped shape shown in FIG. 10. However, the sum of the phasefunctions has significant curvature and therefore the dispersion ishigh, as illustrated in FIG. 10.

In contrast to FIG. 9, FIG. 5 shows the two phase functions Φ1 and Φ2 ofthe two arms in wavelength router 10. The “bending”, or nonlinearbehavior, of the two phase functions are caused by the resonators 30 aand 30 b respectively. It can be seen that the bending direction of boththe phase functions reverse themselves every 25 GHz. Since theresonators 30 a and 30 b have a center frequency difference of 25 GHz,the two phase functions Φ1 and Φ2 have opposite bending directions atany given frequency. The summation of the two phase functions canceleach other's non-linearity, therefore (Φ1+Φ2) has nearly linearcharacteristics as shown in FIG. 5.

From the previously stated properties of the interferometer 100, analmost linear (Φ1+Φ2) function gives low chromatic dispersion. It isequally important to note that the difference of Φ1 and Φ2 remains astep-like function as shown in FIG. 5. As a result, the output waveformhas flat passband and good isolation. The corresponding waveform, groupdelay, and dispersion of one of the two output ports are illustrated inFIG. 6. Note that the group delay and dispersion values in FIG. 6 aremuch smaller compared to the values shown in FIG. 10.

The previous discussion shows a step-by-step construction of awavelength router 10 that performs dispersion-compensation. This is doneto give a quantitative example of router 10. Other channel spacings(e.g., ranging from 12.5 GHz to 100 GHz) can be implemented by changingd1, d2, and ΔL in the spirit described above. By varying the resonatorreflectivities r1 and r2, devices with a passband shape and dispersiondifferent from those of FIG. 6 can also be obtained. A technicaladvantage of router 10 is that no matter what reflectivities r1 and r2are chosen for resonators 30 a and 30 b, the waveform is approximatelysymmetric. This means that the waveform of one output signal has aboutthe same shape as that of the other output signal. The two outputwaveforms are shifted from each other in wavelength, since they arecomplimentary to each other in wavelength space.

In contrast to interferometer 100 illustrated in FIG. 8, wavelengthrouter 10 performs a multiplexing and/or demultiplexing function withvery low chromatic dispersion by employing a structure in which thephase difference function remains step-like, but the phase summationfunction becomes approximately linear with frequency. As a result, thewaveform has a flat-top passband, good isolation, and dispersion issmall.

As is evident in FIG. 5, the phase functions Φ1 and Φ2 have oppositebending characteristics, so their difference forms a step-like curvewhich gives rise to a flat-top waveform. However, because of theopposite bending characteristics of these phase functions, the sum ofthe two functions approximates a straight line and thereby results inlow dispersion.

FIG. 11 illustrates another embodiment of an optical wavelength router1100. The input signal 12 initially passes through a beam displacer 1101which comprises, for example, a birefringent element made from amaterial such as calcite, rutile, lithium niobate, YVO₄-based crystals,and the like. Beam displacer 1101 splits the input signal 12 into twobeams having orthogonal polarizations (e.g., along the X and Ydirections, respectively). A polarization rotator 1102 (e.g., ahalf-wave plate) rotates the polarization of one of the beams by 90degrees, so that both beams have substantially the same polarization.The beam pair then passes through a polarized beamsplitter (PBS) 1103.

The beams are then incident onto a non-reciprocal element, such as aFaraday rotator 1105, which is used to separate the back-propagatingbeams from the polarized input beams propagating in the forwarddirection. Referring to FIGS. 12A and 12B, the Faraday rotator 1105 maybe a magneto-optic element such as a doped garnet crystal 1201 (e.g.,YIG) bonded to a half-wave plate 1202. The crystal 1201 rotates theinput polarization by 45 degrees and the half-wave plate 1202 has itsoptical axis at 22.5 degrees. Thus, the Faraday rotator 1105 transmitslight in the forward direction without changing its polarization, asshown in FIG. 12A, but rotates the polarization of any light from theopposite direction by a predetermined degree (e.g., 90 degrees), asshown in FIG. 12B. Referring back to FIG. 11, the Faraday rotator 1105transmits the polarized input beam pair in the forward direction withoutchanging their polarization, but rotates the polarization of thereflected beam pair from the opposite direction by 90 degrees.

The input beam pair exiting the Faraday rotator 1105 in the forwarddirection then passes through a second PBS 1107. A zero-order beamdisplacer 1115 splits the beams into two pair of orthogonally-polarizedbeams. Various embodiments of a zero-order beam displacer 1115 aredescribed in U.S. patent application Ser. No. 09/547,812, which isincorporated herein by reference. A first pair of beams having a commonpolarization passes through a delay element 1120, such as a block ofglass having a predetermined thickness, and is then reflected back by aresonator 1130 a. A second pair of beams having a polarizationorthogonal to that of the first pair of beams is reflected back by aresonator 1130 b. Resonators 1130 a and 1130 b may comprise the sametype of resonators as resonators 30 a and 30 b described above. Theresonators 1130 a and 1130 b reflect back substantially all of theincident optical power regardless of wavelength, but the group delay ofthe reflected light is strongly dependent on wavelength.

The two pairs of reflected beams from the resonators 1130 a-b arerecombined by back-propagation through the beam displacer 1115 andinterfere to produce one beam pair again. Due to the birefringence ofthe beam displacer 1115, a difference in the optical path lengthsbetween the two beam pairs is generated. As a result, the polarizationstate of the back-propagating beam pair exiting the beam displacer 1115is a function of optical wavelength. In other words, thisback-propagating beam pair has mixed polarization as a function of theoptical wavelengths carried by the beams.

The second PBS 1107 splits this beam pair into two orthogonalpolarizations. One polarization component of each beam is reflected bythe second PBS 1107 and is directed toward output port A. In particular,one of the beams reflected by the second PBS 1107 passes through apolarization rotator 1108 (e.g., a half-wave plate), which rotates thebeam polarization by 90 degrees so that the beam pair entering thebirefringent element 1109 are orthogonally polarized. The birefringentelement 1109 associated with output port A combines theseorthogonally-polarized beams to produce an output signal 22 containing apredetermined subset of the input spectral band.

The other polarization component of each beam is transmitted through thesecond PBS 1107 toward the Faraday rotator 1105 along the same opticalpaths as the polarized input beams, but in the opposite direction. Thepolarization of the beam pair from the second PBS 1107 is rotated by 90degrees by the Faraday rotator 1105, as previously discussed, so thatthey will be separated from the polarized input beams and reflected bythe first PBS 1103 toward output port B. One of the beams reflected bythe first PBS 1103 passes through a polarization rotator 1118 (e.g., ahalf-wave plate), which rotates the beam polarization by 90 degrees sothat the beam pair entering the birefringent element 1119 areorthogonally polarized. The birefringent element 1119 associated withoutput port B combines these orthogonally-polarized beams to produce anoutput signal 24 containing a complementary subset of the input spectralbeam.

FIG. 13 illustrates yet another embodiment of an optical wavelengthrouter 1300 according to the present invention. A portion of router 1300is similar to that of router 1100. Those elements of router 1300 thatdiffer from those of router 1100 will be described in further detail.After the second PBS 1107, the beam pair is horizontally polarized alongthe X axis. A half-wave plate 1301 with its optical axis at 22.5 degreesfrom the X axis rotates the polarization of the beam pair by 45 degrees.A third PBS 1305 splits both beams into two different paths. Thehorizontally polarized components of the beam pair are transmittedthrough the third PBS 1305 and are reflected by the resonator 1130 b, aspreviously described. The vertically polarized components of the beampair are reflected by the third PBS 1305. They pass through the delayelement 1120 and are reflected back by resonator 1130 a. The remainderof this embodiment operates in a manner similar to the embodiment shownin FIG. 11. If desired, one or more waveplates 1303 with optical axes at45 degrees to the X axis can be inserted between the second PBS 1107 andthe third PBS 1305 to allow fine tuning (e.g., by angle) of theinterferometer's path length.

In operation of wavelength routers 1100 and 1300 implementing amultiplexing function, each of birefringent elements 1109 and 1119receives an input signal and splits each respective input signal intobeam pairs having orthogonal polarizations. Polarization rotator 1108rotates one of the beam polarizations of a first beam pair so that bothcomponents of the first beam pair have the same polarization.Polarization rotator 1118 rotates one of the beam polarizations of asecond beam pair so that both components of the second beam pair havethe same polarization. The polarization of the second beam pair may ormay not be the same as that of the first beam pair. PBS 1103 directs thesecond beam pair toward Faraday rotator 1105. When routers 1100 and 1300perform a multiplexing function, the position of rotator 1105 isreversed to the position of rotator 1105 when routers 1100 and 1300 areperforming the demultiplexing function described above with regard toFIGS. 11-13. The first beam pair and the second beam pair interfere andcombine at PBS 1107 to produce one beam pair.

Referring to FIG. 11, zero-order beam displacer 1115 of wavelengthrouter 1100 splits the beams into two pair of orthogonally-polarizedbeams. One pair of orthogonally-polarized beams passes through delayelement 1120 and is then reflected back by a resonator 1130 a. The otherbeam pair is reflected back by a resonator 1130 b. The resonators 1130 aand 1130 b reflect substantially all of the incident optical power backregardless of wavelength, but the group delay of the reflected light isstrongly dependent on wavelength.

The two pairs of reflected beams from the resonators 1130 a-b arerecombined by back-propagation through the beam displacer 1115 andinterfere to produce one beam pair again. Due to the birefringence ofthe beam displacer 1115, a difference in the optical path lengthsbetween the two beam pairs is generated. As a result, the polarizationstate of the back-propagating beam pair exiting the beam displacer 1115is a function of optical wavelength. In other words, thisback-propagating beam pair has mixed polarization as a function of theoptical wavelengths carried by the beams.

Referring to FIG. 13, after passing through PBS 1107, the beam pair ishorizontally polarized along the X axis. Half-wave plate 1303 with itsoptical axis at 22.5 degrees from the X axis rotates the polarization ofthe beam pair by 45 degrees.

PBS 1305 splits both beams into two different paths. The horizontallypolarized components of the beam pair are transmitted through PBS 1305and are reflected by the resonator 1130 b, as previously described. Thevertically polarized components of the beam pair are reflected by PBS1305 and pass through the delay element 1120 after which they arereflected back by resonator 1130 a. The two pairs of reflected beamsfrom the resonators 1130 a-b are recombined by back-propagation throughthe PBS 1305 and interfere to produce one beam pair again. If desired,one or more waveplates 1301 with optical axes at 45 degrees to the Xaxis can be inserted between the second PBS 1107 and the third PBS 1305to allow fine tuning (e.g., by angle) of the interferometer's pathlength.

In both wavelength router 1100 and 1300, PBS 1107, rotator 1105 and PBS1103 direct the back-propagating beam pair to birefringent element 1101.The polarization of one component of the beam pair is rotated by ninetydegrees by polarization rotator 1102 so that the beam pair entering thebirefringent element 1101 is orthogonally polarized. Birefringentelement 1101 combines these orthogonally polarized beams to produce amultiplexed output signal.

It should be understood that the use of resonators 1130 a and 1130 b inwavelength routers 1100 and 1300 results in low chromatic dispersion, asdescribed above with regard to wavelength router 10. Therefore, theperformance characteristics illustrated in FIGS. 5 and 6 with regard towavelength router 10 generally apply to wavelength routers 1100 and 1300as well. As a result, routers 1100 and 1300 comprise alternativeembodiments of router 10, but each of routers 10, 1100, and 1300performs demultiplexing and/or multiplexing functions while achievinglow chromatic dispersion.

FIG. 14 illustrates a cascaded architecture 1400 of optical filters. Forexample, a first stage of architecture 1400 may include an opticalfilter 1402. A second stage of architecture 1400 may include opticalfilters 1410 a and 1410 b, which are collectively referred to as opticalfilters 1410. Third stage of architecture 1400 may include opticalfilters 1420 a, 1420 b, 1420 c, and 1420 d, which are collectivelyreferred to as optical filters 1420. Optical filters 1402, 1410, and1420 may comprise any combination and arrangement of optical filtersthat employ any suitable conventional optical filtering technology(e.g., fiber bragg gratings, thin film filters, arrayed waveguidegrating, etc.) and optical wavelength routers 10, 1100, and 1300described above.

In the particular embodiment illustrated in FIG. 14, filter 1402comprises a 50 GHz optical router 10 that receives a 50 GHz spaced densewavelength division multiplexed (DWDM) signal 1405 and generates anodd-channel 100 GHz spacing DWDM signal 1415 and an even channel 100 GHzspacing signal 1417. Two 100 GHz filters 1410 a and 1410 b are used toproduce a 200 GHz spaced signal 1431 carrying wavelengths λ₁ and λ₅, asignal 1429 carrying wavelengths λ₃ and λ₇, a signal 1427 carryingwavelengths λ₂ and λ₆, and a signal 1425 carrying wavelengths λ₄ and λ₈.A third stage of filters 1420 a-d are used to produce the individualchannels λ₁ through λ₈ on outputs 1441, 1449, 1445, 1453, 1443, 1451,1447, and 1455 respectively. Signals 1415, 1417, 1425, 1427, 1429, and1431 may be referred to as intermediate input signals and/orintermediate output signals with respect to a particular filter 1402,1410, or 1420. By using one or more optical wavelength routers 10, 1100,and 1300 in the cascaded architecture 1400, the device significantlyreduces chromatic dispersion while keeping a flat-top passband for eachchannel and good isolation among channels.

Although FIG. 14 illustrates architecture 1400 having three stages offilters to demultiplex a DWDM signal 1405 having eight wavelengthchannels, it is contemplated that architecture 1400 may have anysuitable number of stages to demultiplex a DWDM signal 1405 having anysuitable number of wavelength channels. Moreover, FIG. 14 is detailedwith respect to demultiplexing a 50 GHz spaced DWDM signal 1405 forillustrative purposes only. It is contemplated that a DWDM signal 1405having any suitable channel spacing (12.5 GHz, 50 GHz, 100 GHz, 200 GHz,etc.) may be processed by the architecture 1400 of filters.Additionally, although the description of architecture 1400 is detailedwith respect to a demultiplexing function, it should be understood thatit can also perform a multiplexing function upon individual wavelengthchannels to produce one or more DWDM signals while achieving lowchromatic dispersion.

FIG. 15 illustrates one embodiment of an optical networking architecture1500 that includes an optical network 1505 coupled to a demultiplexernetwork 1510, filters 1550, switch fabrics 1560, regulators 1570,filters 1580, and a multiplexer network 1530. In general, opticalwavelength routers 10, 1100, and/or 1300 may be incorporated intoarchitecture 1500, such as in demultiplexer network 1510 and/ormultiplexer network 1530, to compensate for chromatic dispersion. Itshould be understood that architecture 1500 may be configureddifferently and/or may include additional or fewer components withoutdeparting from the scope of the present invention.

Optical network 1505 comprises any combination and arrangement ofrouters, bridges, hubs, gateways, switches, multiplexers,demultiplexers, transmitters, amplifiers, receivers, couplers,isolators, circulators, filters, detectors, wavelength converters,add/drop devices, or any other appropriate optical networkingcomponents. Optical network 1505 may include portions of a long-haulnetwork, a metropolitan network, and/or a local/access network.

Demultiplexer network 1510 and multiplexer network 1530 each comprise anappropriate arrangement of filters. For example, demultiplexer network1510 comprises filters 1512, 1514, 1516, 1518, and 1520. One or more offilters 1512-1520 may comprise a wavelength router 10, 1100, and/or 1300to perform a demultiplexing function while compensating for chromaticdispersion. Similarly, multiplexer network 1530 may comprise filters1532, 1534, 1536, 1538, and 1540. One or more of filters 1532-1540 maycomprise a wavelength router 10, 1100, and/or 1300 to perform amultiplexing function while compensating for chromatic dispersion.

Filters 1550 and 1580 comprise gratings, Bragg gratings, Fiber gratings,Fiber Bragg gratings, Fabry-Perot filters, Thin-Film filters,interferometers, arrayed waveguide gratings, tunable filters, or anyother optical device that process and differentiate among opticalsignals based upon wavelength.

Switch fabrics 1560 comprise switches and/or routers. In one embodimentswitch fabrics 1560 comprise add/drop switch arrays. Various embodimentsof an add/drop switch array are disclosed in U.S. patent applicationSer. No. 09/273,920, which is incorporated herein by reference.Regulators 1570 comprise any suitable device that adjustably regulatethe optical power level of an optical channel.

In operation, demultiplexer network 1510 receives input signal 12 fromnetwork 1505. Demultiplexer network 1510 and filters 1550 separate inputsignal 12 into an array of spatially separated wavelength channels. Thisis generally done by progressively demultiplexing input signal 12 intointermediate signals, such as, for example, intermediate signals 1522a-b, 1524 a-b, 1526, 1528 a-b, and 1529 a-b which may be referred to asintermediate input signals and/or intermediate output signals withrespect to a particular filter 1512, 1514, 1516, 1518, and 1520. Byusing one or more optical routers 10, 1100, and/or 1300 in demultiplexernetwork 1510, each spatially separated wavelength channel generally hasa flat-top passband, good isolation from other channels, and lowchromatic dispersion. Switch fabrics 1560 process the spatiallyseparated channels to perform a switching and/or routing function. In aparticular embodiment, a switch fabric 1560 may comprise an add/dropswitch array that selectively routes channels from the input ports toits drop ports; substitutes channels from the add ports in place of thedropped channels; and routes the remaining input channels and the addedchannels to the output ports of the add/drop switch array. Thiscombination of demultiplexer network 1510, filters 1550 and add/dropswitch arrays 1560 allows any combination of input channels to bereplaced with any combination of add channels.

In one embodiment, the array of output channels from the switch fabrics1550 pass through regulators 1570 which adjustably regulate the opticalpower level of each channel. In a particular embodiment, a selectedsubset of the channels associated with input signal 12 pass directlyfrom demultiplexer network 1510 to multiplexer network 1530 in “expresslanes.” A second array of filters 1580 and a multiplexing network 1530combine the output channels so that they can be transmitted as a DWDMoutput signal 1590. This is generally done by progressively multiplexinginto output signal 1590 intermediate signals, such as, for example,intermediate signals 1542 a-b, 1544 a-b, 1546 a-b, 1548, and 1549 a-bwhich may be referred to as intermediate input signals and/orintermediate output signals with respect to a particular filter 1532,1534, 1536, 1538, and 1540. By using one or more optical routers 10,1100, and/or 1300 in multiplexer network 1530, the wavelength channelscomprising output signal 1590 generally have low chromatic dispersion.

FIGS. 16A and 16B illustrate the dispersion profiles of modifiedembodiments of wavelength router 10. In a first modified embodiment ofrouter 10, referred to hereinafter as router 1600, the optical thicknessd2 of resonator 30 b is selected so that resonator 30 b has a freespectral range (FSR) of approximately 50 GHz and the resonancefrequencies are at f_(c)−6.25 GHz. Here f_(c) denotes the centerfrequencies of the WDM channels of input signal 12 that are spaced, forexample, 50 GHz apart. The FSR of resonator 30 b here is defined as theperiod of the resonator's complex reflectivity.

The above conditions are achieved by following the equation:d2=(m/2)*λ_(c)+({fraction (1/16)})_(.)*λ_(c)and picking the integer m such that the equation:d2=c/(2*FSR)is satisfied to best approximation. Here, λ_(c) is the center wavelengthof any one of the input channels within the FSR of the particularresonator 30; and c is the speed of light in a vacuum. In a particularembodiment, λ_(.c) is the center wavelength of the center input channelwithin the FSR of the particular resonator 30. In an example for ac-band 50 GHz router 10, we can use λ_(.c)=1545.32 nm, c=2.99792458*10⁸m/sec, and therefore d2=2.998017 mm. The optical thickness d1 ofresonator 30 a is set such that d1=d2−(⅛)_(.)*λ_(c). In the examplehere, d1=2.997824 mm.

The dispersion profile of router 1600 is illustrated in FIG. 16A. Notethat each period of the chromatic dispersion profile has a positiveslope for a first range of frequencies and a negative slope for a secondrange of frequencies.

The second modified embodiment of the router 10, referred to hereinafteras router 1610, can be constructed by exchanging the optical thicknessesof resonators 30 a and 30 b. For example, router 1600 yielding theperformance characteristic illustrated in FIG. 16A can have resonator 30a with optical thickness, d1=2.997824, and resonator 30 b with opticalthickness, d2=2.998017, as determined above. Router 1610 yielding thedispersion profile illustrated in FIG. 16B can have resonator 30 a withoptical thickness, d1=2.998017, and resonator 30 b with opticalthickness, d2=2.997824. Note that each period of the chromaticdispersion profile has a negative slope for a first range of frequenciesand a positive slope for a second range of frequencies. Therefore, byexchanging the optical thicknesses of resonators 30 a and 30 b fromrouter 1600 to router 1610, the chromatic dispersion profile is invertedabout the center frequency along the x-axis and inverted about the zerodispersion measurement about the y-axis.

By following the procedure above, the center wavelength of the resonator30 a is offset relative to the center wavelength of the resonator 30 bby approximately one-quarter of the free spectral range of both theresonators. For example, if the free spectral range of both resonatorsis approximately 50 GHz, then the center wavelength of resonator 30 a isoffset by approximately 12.5 GHz relative to the center wavelength ofresonator 30 b. As will be elaborated further below, this arrangement ofthe resonators' center wavelengths reduce chromatic dispersion in amulti-stage optical wavelength router configuration, while keeping aflat-top passband and good isolation.

The back mirror reflectivities of the two resonators 30 are both set tobe 100%. The front mirror reflectivities of the resonators 30 can bevaried to adjust the passband, isolation, and dispersion of theinterleaver waveform. In the example of FIGS. 16A and 16B, the frontmirror reflectivities are set at r2=0.2 (i.e., 4% reflectivity) andr1=0.2. Finally, the interferometer path length difference, ΔL, is setto be approximately (½)*d2, which comes out to ΔL≈1.499 mm. Note thatthe exact ΔL values for routers 1600 and 1610 can differ by a smallamount that is a fraction of a wavelength.

Referring back to FIG. 14, the cascaded architecture 1400 of opticalfilters may further include an arrangement of routers 10, 100, 1100,1300, 1600, and 1610 to yield a reduction in chromatic dispersion forthe individual wavelength channels, λ₁ through λ₈. Although thefollowing description of cascaded architecture 1400 is described withreference to a two-stage configuration operating as a demultiplexer, itshould be understood that the arrangement of routers 10, 100, 1100,1300, 1600, and 1610 in cascaded architecture 1400 can yield a reductionin chromatic dispersion for individual wavelength channels λ₁ through λ₈for a configuration having any number of stages greater than one whenoperating as a demultiplexer or a multiplexer.

In one embodiment of architecture 1400, a first stage of architecture1400 includes a router 10 characterized by a chromatic dispersionprofile having a first frequency offset and a second stage ofarchitecture 1400 includes at least one router 10 characterized by achromatic dispersion profile having a second frequency offset. Thechromatic dispersion profile of each of the first stage, second stage,and combined stages is illustrated in greater detail with reference toFIG. 17. In particular, spectral curve 1710 corresponds to the chromaticdispersion profile of the first stage router 10 with a first frequencyoffset; spectral curve 1712 corresponds to the chromatic dispersionprofile of the second stage router 10 with a second frequency offset;and spectral curve 1714 corresponds to the chromatic dispersion profileof the combined first and second stage routers 10. Note that spectralcurves 1710 and 1712 each approach a sinusoidal curve as thereflectivities r1 and r2 of resonators 30 a and 30 b, respectively,become substantially the same.

With reference to a 50 GHz router 10, for example, the first frequencyoffset may be such that for each individual wavelength channel theΔf_(c)=+6.25 GHz, and the second frequency offset may be such that foreach individual wavelength channel, the Δf_(c)=−6.25 GHz. In anotherexample, the first frequency offset may be such that for each individualwavelength channel the Δf_(c)=−6.25 GHz, and the second frequency offsetmay be such that for each individual wavelength channel the Δf_(c)=+6.25GHz.

In yet another example, the first frequency offset may be such that foreach individual wavelength channel the Δf_(c)=0 GHz, and the secondfrequency offset may be such that for each individual wavelength channelthe Δf_(c)=+/−12.50 GHz. In still another example, the first frequencyoffset may be such that for each individual wavelength channel theΔf_(c)=+/−12.5 GHz, and the second frequency offset may be such that foreach individual wavelength channel the Δf_(c)=0 GHz. In this regard, therelative center frequency shift between the two stages is approximatelyone-half of the period of the chromatic dispersion profile. As a result,the chromatic dispersion profile of the first stage is almost oppositeto that of the second stage at all frequencies, which allows them tocancel each other when the first and second stages are cascaded. Thefirst and second frequency offsets may be achieved by a number ofdifferent techniques, such as, for example, (1) by changing thetemperature in the cavity of any given resonator 30 to offset the centerfrequency of that resonator 30 by a predetermined amount; (2) byinserting and adjusting the angle of a transparent plate along theoptical path within the cavity of any given resonator 30 to offset thecenter frequency of that resonator 30 by a predetermined amount; and (3)by changing the air pressure in the cavity of any given resonator 30 tooffset the center frequency of that resonator 30 by a predeterminedamount.

In another embodiment of architecture 1400, a first stage ofarchitecture 1400 includes a router 100 characterized by a chromaticdispersion profile having a first slope at the center frequency of anyparticular wavelength channel, and a second stage of architecture 1400includes at least one router 10 characterized by a chromatic dispersionprofile having a second slope at the center frequency of the particularwavelength channel that is substantially opposite to the first slope.The chromatic dispersion profile of each of the first stage, secondstage, and combined stages is illustrated in greater detail withreference to FIG. 18. In particular, spectral curve 1810 corresponds tothe chromatic dispersion profile of the first stage router 100; spectralcurve 1812 corresponds to the chromatic dispersion profile of the secondstage router 10; and spectral curve 1814 corresponds to the chromaticdispersion profile of the combined first and second stage routers 100and 10. In this regard, the slope of the chromatic dispersion profile ofthe second stage optical wavelength router 10 is substantially oppositeto the slope of the chromatic dispersion profile of the first stageoptical wavelength router 10 over a range of frequencies surrounding thecenter frequency of any particular wavelength channel. As a result, whenthe first and second stages are cascaded, the chromatic dispersionprofile of the second stage router 100 substantially compensates thechromatic dispersion profile of the first stage router 10 over a rangeof frequencies surrounding the center frequency of any particularwavelength channel. Although this embodiment of architecture 1400 isdescribed with reference to the first stage router 100 and the secondstage router 10, it should be understood that architecture 1400 may alsobe arranged having a first stage router 10 and a second stage router 100to accomplish chromatic dispersion reduction.

In another embodiment of architecture 1400, a first stage ofarchitecture 1400 includes a router 1600 and the second stage includesat least one router 1610. The chromatic dispersion profile of each ofthe first stage, second stage, and combined stages is illustrated ingreater detail with reference to FIG. 19. In particular, spectral curve1910 corresponds to the chromatic dispersion profile of the first stagerouter 1600 having a first slope over a first range of frequencies of aparticular wavelength channel and a second slope over a second range offrequencies of the particular wavelength channel. The second slope issubstantially opposite to the first slope. Spectral curve 1912corresponds to the chromatic dispersion profile of the second stagerouter 1610 having the second slope over the first range of frequenciesof the particular wavelength channel and the first slope over the secondrange of frequencies of the particular wavelength channel. Spectralcurve 1914 corresponds to the chromatic dispersion profile of thecombined first and second stage routers 1600 and 1610. In this regard,when the first and second stages are cascaded, the chromatic dispersionprofile associated with the second stage router 1610 substantiallycompensates the chromatic dispersion profile associated with the firststage router 1600 over the first and second range of frequencies of theparticular wavelength channel, yielding the chromatic dispersion profileillustrated by spectral curve 1914. Although this embodiment ofarchitecture 1400 is described with reference to the first stage router1600 and the second stage router 1610, it should be understood thatarchitecture 1400 may also be arranged having a first stage router 1610and a second stage router 1600 to accomplish chromatic dispersionreduction.

The above disclosure sets forth a number of embodiments of the presentinvention. Other arrangements or embodiments, not precisely set forth,could be practiced under the teachings of the present invention and asset forth in the following claims.

1. An optical wavelength router comprising: a beamsplitter operable toseparate an input signal into a first beam and a second beam; a firstresonator operable to reflect the first beam and having a partiallyreflective front surface and a highly reflective back surface spaced afirst optical thickness from the front surface; and a second resonatoroperable to reflect the second beam and having a partially reflectivefront surface and a highly reflective back surface spaced a secondoptical thickness from the front surface; wherein the difference betweenthe first optical thickness and the second optical thickness isapproximately equal to one-eighth wavelength.
 2. The router of claim 1,wherein the beamsplitter is further operable to process the first beamand the second beam to generate a first output signal for communicationto a first output port and to generate a second output signal forcommunication to a second output port, the first output signalcomprising a first subset of channels from the input signal and thesecond output signal comprising a second subset of channels from theinput signal.
 3. The router of claim 2, wherein: the input signalcomprises an input spectral band; the first output signal comprises afirst subset of the input spectral band; and the second output signalcomprises a second subset of the input spectral band that iscomplementary to the first subset of the input spectral band.
 4. Therouter of claim 1, wherein: the input signal comprises a first inputsignal; the beamsplitter is operable to separate a second input signalinto a third beam and a fourth beam; the first resonator is furtheroperable to reflect the third beam; the second resonator is furtheroperable to reflect the fourth beam; the beamsplitter is furtheroperable to process the first beam, the second beam, the third beam, andthe fourth beam to generate an output signal for communication to anoutput port; and the output signal comprises channels of the first inputsignal combined with channels of the second input signal.
 5. The routerof claim 1, wherein: the first resonator has a first center wavelength;and the second resonator has a second center wavelength; and the secondcenter wavelength is offset relative to the first center wavelength byapproximately one quarter of the free spectral range of the firstresonator.
 6. The router of claim 1, wherein: the first beam propagatesalong an optical path having a first optical path length; the secondbeam propagates along an optical path having a second optical pathlength; and the difference between the first optical path length and thesecond optical path length is approximately equal to one half of thefirst optical thickness.
 7. An optical device, comprising: a first stageoptical wavelength router that receives an input wavelength divisionmultiplexed signal and that generates a first output signal comprising afirst subset of wavelength channels from the input signal and a secondoutput signal comprising a second subset of wavelength channels from theinput signal, wherein the first stage optical wavelength router ischaracterized by a chromatic dispersion profile having a first frequencyoffset; and a second stage optical wavelength router that receives thefirst output signal and that generates a third output signal and afourth output signal, wherein the second stage optical wavelength routeris characterized by the chromatic dispersion profile having a secondfrequency offset such that the difference between the first frequencyoffset and the second frequency offset comprises one-half of the periodof the chromatic dispersion profile.
 8. The optical device of claim 7,wherein the first stage optical wavelength router comprises: abeamsplitter that separates the input signal into a first beam and asecond beam; a first resonator that reflects the first beam and having apartially reflective front surface and a highly reflective back surfacespaced a first optical thickness from the front surface; and a secondresonator that reflects the second beam and having a partiallyreflective front surface and a highly reflective back surface spaced asecond optical thickness from the front surface; wherein the differencebetween the first optical thickness and the second optical thickness isapproximately equal to one-quarter wavelength.
 9. The optical device ofclaim 7, wherein the second stage optical wavelength router comprises: abeamsplitter that separates the first output signal into a first beamand a second beam; a first resonator that reflects the first beam andhaving a partially reflective front surface and a highly reflective backsurface spaced a first optical thickness from the front surface; and asecond resonator that reflects the second beam and having a partiallyreflective front surface and a highly reflective back surface spaced asecond optical thickness from the front surface; wherein the differencebetween the first optical thickness and the second optical thickness isapproximately equal to one-quarter wavelength.
 10. The optical device ofclaim 8, wherein the first resonator and the second resonator each haveapproximately the same front mirror reflectivity.
 11. The optical deviceof claim 7, wherein: the chromatic dispersion profile of the first stageoptical wavelength router is offset in a first direction by a particularfrequency; and the chromatic dispersion profile of the second stageoptical wavelength router is offset by the particular frequency in asecond direction substantially opposite to that of the first direction.12. The optical device of claim 7, wherein: the magnitude of the firstfrequency offset is substantially zero; and the magnitude of the secondfrequency offset is substantially one-half of the period of thechromatic dispersion profile.
 13. The optical device of claim 7, whereinthe chromatic dispersion profile of the second stage optical wavelengthrouter is substantially opposite to the chromatic dispersion profile ofthe first stage optical wavelength router.
 14. An optical device,comprising: a first stage optical wavelength router that receives afirst input signal comprising a first subset of wavelength channels anda second input signal comprising a second subset of wavelength channels,and that generates a first output signal comprising the first and secondsubsets of wavelength channels, wherein the first stage opticalwavelength router is characterized by a chromatic dispersion profilehaving a first frequency offset; and a second stage optical wavelengthrouter that receives the first output signal and a third input signalcomprising a third subset of wavelength channels, and that generates asecond output signal comprising the first, second, and third subsets ofwavelength channels, wherein the second stage optical wavelength routeris characterized by the chromatic dispersion profile having a secondfrequency offset such that the difference between the first frequencyoffset and the second frequency offset comprises one-half of the periodof the chromatic dispersion profile.
 15. The optical device of claim 14,wherein the first stage optical wavelength router comprises: abeamsplitter; a first resonator having a partially reflective frontsurface and a highly reflective back surface spaced a second opticalthickness from the front surface; and a second resonator having apartially reflective front surface and a highly reflective back surfacespaced a second optical thickness from the front surface; wherein thedifference between the first optical thickness and the second opticalthickness is approximately equal to one-quarter wavelength.
 16. Theoptical device of claim 14, wherein the second stage optical wavelengthrouter comprises: a beamsplitter; a first resonator having a partiallyreflective front surface and a highly reflective back surface spaced asecond optical thickness from the front surface; and a second resonatorhaving a partially reflective front surface and a highly reflective backsurface spaced a second optical thickness from the front surface;wherein the difference between the first optical thickness and thesecond optical thickness is approximately equal to one-quarterwavelength.
 17. The optical device of claim 14, wherein: the chromaticdispersion profile of the first stage optical wavelength router isoffset in a first direction by a particular frequency; and the chromaticdispersion profile of the second stage optical wavelength router isoffset by the particular frequency in a second direction substantiallyopposite to that of the first direction.
 18. The optical device of claim14, wherein: the magnitude of the first frequency offset issubstantially zero; and the magnitude of the second frequency offset issubstantially one-half of the period of the chromatic dispersionprofile.
 19. The optical device of claim 14, wherein the chromaticdispersion profile of the second stage optical wavelength router issubstantially opposite to the chromatic dispersion profile of the firststage optical wavelength router.
 20. An optical device, comprising: afirst stage optical wavelength router operable to receive an inputwavelength division multiplexed signal and to generate a first outputsignal comprising a first subset of wavelength channels from the inputsignal and a second output signal comprising a second subset ofwavelength channels from the input signal, wherein the first stageoptical wavelength router is characterized by a chromatic dispersionprofile having a first slope at a center frequency of a particularwavelength channel; and a second stage optical wavelength routeroperable to receive the first output signal and to generate a thirdoutput signal and a fourth output signal, wherein the second stageoptical wavelength router is characterized by a chromatic dispersionprofile having a second slope at the center frequency that issubstantially opposite to the first slope.
 21. The optical device ofclaim 20, wherein the chromatic dispersion profile of the second stageoptical wavelength router is substantially opposite to the chromaticdispersion profile of the first stage optical wavelength router over arange of frequencies surrounding the center frequency.
 22. The opticaldevice of claim 20, wherein the first stage optical wavelength routercomprises: a beamsplitter operable to separate an input signal into afirst beam and a second beam; a mirror operable to reflect the firstbeam; and a resonator operable to reflect the second beam and having apartially reflective front surface and a highly reflective back surface.23. The optical device of claim 20, wherein the second stage opticalwavelength router comprises: a beamsplitter operable to separate aninput signal into a first beam and a second beam; a first resonatoroperable to reflect the first beam and having a partially reflectivefront surface and a highly reflective back surface spaced a firstoptical thickness from the front surface; and a second resonatoroperable to reflect the second beam and having a partially reflectivefront surface and a highly reflective back surface spaced a secondoptical thickness from the front surface; wherein the difference betweenthe first optical thickness and the second optical thickness isapproximately equal to one-quarter wavelength.
 24. An optical device,comprising: a first stage optical wavelength router operable to receivea first input signal comprising a first subset of wavelength channelsand a second input signal comprising a second subset of wavelengthchannels, and further operable to generate a first output signalcomprising the first and second subsets of wavelength channels, whereinthe first stage optical wavelength router is characterized by achromatic dispersion profile having a first slope at a center frequencyof a particular wavelength channel; and a second stage opticalwavelength router operable to receive the first output signal and athird input signal comprising a third subset of wavelength channels, andfurther operable to generate a second output signal comprising thefirst, second, and third subsets of wavelength channels, wherein thesecond stage optical wavelength router is characterized by a chromaticdispersion profile having a second slope at the center frequency that issubstantially opposite to the first slope.
 25. The optical device ofclaim 24, wherein the chromatic dispersion profile of the second stageoptical wavelength router is substantially opposite to the chromaticdispersion profile of the first stage optical wavelength router over arange of frequencies surrounding the center frequency.
 26. The opticaldevice of claim 24, wherein the first stage optical wavelength routercomprises: a beamsplitter; a first resonator having a partiallyreflective front surface and a highly reflective back surface spaced afirst optical thickness from the front surface; and a second resonatorhaving a partially reflective front surface and a highly reflective backsurface spaced a second optical thickness from the front surface;wherein the difference between the first optical thickness and thesecond optical thickness is approximately equal to one-quarterwavelength.
 27. The optical device of claim 24, wherein the second stageoptical wavelength router comprises: a beamsplitter operable to separatean input signal into a first beam and a second beam; a mirror operableto reflect the first beam; and a resonator operable to reflect thesecond beam and having a partially reflective front surface and a highlyreflective back surface.
 28. An optical device, comprising: a firststage optical wavelength router operable to receive an input wavelengthdivision multiplexed signal and to generate a first output signalcomprising a first subset of wavelength channels from the input signaland a second output signal comprising a second subset of wavelengthchannels from the input signal, wherein the first stage opticalwavelength router is characterized by a chromatic dispersion profilehaving a first slope over a first range of frequencies and a secondslope over a second range of frequencies, the second slope beingsubstantially opposite to the first slope; and a second stage opticalwavelength router operable to receive the first output signal and togenerate a third output signal and a fourth output signal, wherein thesecond stage optical wavelength router is characterized by a chromaticdispersion profile having the second slope over the first range offrequencies and the first slope over the second range of frequencies.29. The optical device of claim 28, wherein the chromatic dispersionprofile associated with the second stage optical wavelength router issubstantially opposite to the chromatic dispersion profile associatedwith the first stage optical wavelength router over the first and secondrange of frequencies.
 30. The router of claim 28, wherein the firststage optical wavelength router comprises: a beamsplitter operable toseparate an input signal into a first beam and a second beam; a firstresonator operable to reflect the first beam and having a partiallyreflective front surface and a highly reflective back surface spaced afirst optical thickness from the front surface; and a second resonatoroperable to reflect the second beam and having a partially reflectivefront surface and a highly reflective back surface spaced a secondoptical thickness from the front surface; wherein the difference betweenthe first optical thickness and the second optical thickness isapproximately equal to one-eighth wavelength.
 31. The router of claim28, wherein the second stage optical wavelength router comprises: abeamsplitter operable to separate an input signal into a first beam anda second beam; a first resonator operable to reflect the first beam andhaving a partially reflective front surface and a highly reflective backsurface spaced a first optical thickness from the front surface; and asecond resonator operable to reflect the second beam and having apartially reflective front surface and a highly reflective back surfacespaced a second optical thickness from the front surface; wherein thedifference between the first optical thickness and the second opticalthickness is approximately equal to one-eighth wavelength.
 32. Anoptical device, comprising: a first stage optical wavelength routeroperable to receive a first input signal comprising a first subset ofwavelength channels and a second input signal comprising a second subsetof wavelength channels, and further operable to generate a first outputsignal comprising the first and second subsets of wavelength channels,wherein the first stage optical wavelength router is characterized by achromatic dispersion profile having a first slope over a first range offrequencies and a second slope over a second range of frequencies, thesecond slope being substantially opposite to the first slope; and asecond stage optical wavelength router operable to receive the firstoutput signal and a third input signal comprising a third subset ofwavelength channels, and further operable to generate a second outputsignal comprising the first, second, and third subsets of wavelengthchannels, wherein the second stage optical wavelength router ischaracterized by a chromatic dispersion profile having the second slopeover the first range of frequencies and the first slope over the secondrange of frequencies.
 33. The optical device of claim 32, wherein thechromatic dispersion profile associated with the second stage opticalwavelength router is substantially opposite to the chromatic dispersionprofile associated with the first stage optical wavelength router overthe first and second range of frequencies.
 34. The router of claim 32,wherein the first stage optical wavelength router comprises: abeamsplitter; a first resonator having a partially reflective frontsurface and a highly reflective back surface spaced a first opticalthickness from the front surface; and a second resonator having apartially reflective front surface and a highly reflective back surfacespaced a second optical thickness from the front surface; wherein thedifference between the first optical thickness and the second opticalthickness is approximately equal to one-eighth wavelength.
 35. Therouter of claim 32, wherein the second stage optical wavelength routercomprises: a beamsplitter; a first resonator having a partiallyreflective front surface and a highly reflective back surface spaced afirst optical thickness from the front surface; and a second resonatorhaving a partially reflective front surface and a highly reflective backsurface spaced a second optical thickness from the front surface;wherein the difference between the first optical thickness and thesecond optical thickness is approximately equal to one-eighthwavelength.